[Paper Review] Near-infrared properties of classical novae: a perspective gained from Mount Abu Infrared Observatory

D. P. K. Banerjee, N. M. Ashok|arXiv (Cornell University)|Jun 3, 2013

Astrophysical Phenomena and Observations2 references10 citations

TL;DR

This paper extends the optical classification of classical novae into the near-infrared (NIR) regime using spectroscopic data from the Mount Abu Infrared Observatory. It identifies carbon lines as key NIR discriminators between Fe II and He/N nova classes, confirms CO emission as a precursor to dust formation, and provides observational limits on the 12C/13C ratio, offering a predictive framework for dust formation in novae.

ABSTRACT

We review the near-infrared properties of classical novae in the J, H and K bands at wavelengths between 1.08 to 2.4 micron. A classification system exists for the early post-outburst optical spectra of novae on the basis of the strength of group of non-hydrogen emission lines. A similar scheme for the near-infrared regime, which is not available at present, is presented here. In the optical system there are two principal classes, namely, "Fe II" and "He/N" for novae with either prominent Fe II lines or prominent "He/N" lines. There is also a small subset of the hybrid Fe IIb type. From spectroscopic observations we show the differences and similarities between these classes of novae in the near-infrared. The spectral lines common to the two principal classes arise from H, He, N and O. However, the near-IR features that separate these two classes are the numerous, and often strong, Carbon lines which are seen only in the spectra of the Fe II class of novae. The dust formation process in novae is discussed based on broad-band observations. The first-overtone carbon monoxide (CO) detections in novae are analyzed to understand the formation and evolution of this molecule in the nova ejecta and to discuss the observed 12C/13C ratio.

Motivation & Objective

• To develop a near-infrared classification system for classical novae analogous to the established optical Fe II/He/N scheme.
• To investigate spectral differences between Fe II and He/N nova classes in the J, H, and K bands (1.08–2.4 µm).
• To analyze the role of CO emission in nova dust formation and assess its predictive value.
• To estimate the 12C/13C ratio in nova ejecta using observed CO band profiles and compare with theoretical models.

Proposed method

• Acquired early post-outburst NIR spectra of 39 novae using the 1.2 m telescope at the Mount Abu Infrared Observatory.
• Analyzed J, H, and K band spectra (1.08–2.35 µm) to identify emission lines and classify novae into Fe II or He/N types based on line strengths.
• Used model fitting of CO first-overtone bands (v=2-0, 3-1, 4-2) to estimate CO parameters and infer 12C/13C ratios, correcting for contamination by CI and NaI lines.
• Correlated CO emission presence with subsequent dust formation observed in light curves to test its predictive power.
• Applied a simple predictive scheme to forecast dust formation based on early CO detection.
• Evaluated the reliability of 12C/13C ratio estimates by assessing the impact of line blending and optical depth assumptions.

Experimental results

Research questions

• RQ1 Can a near-infrared classification system for classical novae be established that parallels the optical Fe II/He/N scheme?
• RQ2 What spectral features in the J, H, and K bands distinguish Fe II-type from He/N-type novae?
• RQ3 Is the detection of CO emission in the first-overtone bands a reliable predictor of dust formation in novae?
• RQ4 What are the observed 12C/13C ratios in nova ejecta, and how do they compare with theoretical predictions?
• RQ5 To what extent do spectral contaminants (CI, NaI) affect the accuracy of CO and 12C/13C ratio measurements?

Key findings

• Carbon lines are prominent and diagnostic in Fe II-type novae in the NIR, distinguishing them from He/N-type novae, which lack strong C features.
• All novae showing detectable CO emission in the K band subsequently formed dust, supporting CO detection as a reliable predictor of dust formation.
• The 12C/13C ratio in novae is constrained to ≥1.5 in V496 Sct and ≥2 in V2615 Oph, with values ranging from ≥1.5 to ≥5 across different novae, though most observed values exceed unity despite theoretical expectations of <1.
• CO band profiles are significantly contaminated by CI (2.2906 µm) and NaI (2.3348, 2.3379 µm) lines, complicating accurate 12C/13C ratio estimation.
• Doppler broadening causes significant overlap between 12CO and 13CO bands, making clear separation difficult in high-velocity nova ejecta.
• In V1280 Sco, despite high signal-to-noise and early monitoring, no CO emission was detected, suggesting that not all dust-producing novae emit detectable CO, possibly due to low or short-lived CO production.